With tightening government regulations on automobile emissions, modern vehicles use a three-way catalyst (TWC) for engine exhaust after-treatment. Ceria is commonly added to the catalyst to chemically store oxygen and help curb emissions breakthrough by increasing the operating window about the stoichiometric air-to-fuel ratio (AFR). The TWC may be maintained at a desired fractional oxidation state (FOS) based on catalyst monitor sensors and/or physics-based catalyst models. For example, some vehicles have a universal exhaust gas oxygen (UEGO) sensor upstream of the TWC and a heated exhaust gas oxygen (HEGO) sensor downstream of the TWC to help maintain the AFR and the FOS of the catalyst at set points. Specifically, the upstream UEGO sensor provides feedback to adjust engine exhaust gases about stoichiometry. The downstream HEGO sensor provides feedback to bias the engine AFR richer or leaner to increase catalyst efficiency. Furthermore, the downstream HEGO sensor may be used for catalyst diagnostics.
Degradation of the HEGO sensor may lead to increased fuel consumption and emissions. One existing intrusive approach for diagnosing HEGO sensor degradation involves actively adjusting engine operation in order to collect data on HEGO sensor performance. For example, engine operation may be actively adjusted to effect one or more rich-to-lean or lean-to-rich transitions, in order to monitor the response of the HEGO sensor to these transitions. Another existing approach, which is non-intrusive, is disclosed in U.S. Pat. No. 5,801,295. Therein, HEGO sensor output voltage is monitored when certain entry conditions are met, and voltage trace segments are summed over a specified period. The length of the summed HEGO voltage trace is compared to a threshold length; a trace length below than the threshold length indicates decreased sensitivity and robustness of the HEGO sensor.
However, the inventors herein have recognized potential issues with such approaches. For example, in the intrusive approach, the required excursions from normal engine operation may be restricted to particular operating conditions that do not occur frequently enough to accurately monitor the sensor. Further, these excursions may increase the duration of engine operation at non-desired AFRs, resulting in increased fuel consumption and/or increased emissions. In the non-intrusive approach, in order to perform the diagnosis, various entry conditions must be met, such that HEGO sensor degradation may remain undetected for an excessive period of time during which it negatively affects engine performance. In addition, as this approach associates decreased frequency of switching alone with HEGO sensor degradation, types of HEGO sensor degradation which do not affect the frequency of switching may go unnoticed.
The inventors herein have identified methods and systems which overcome the deficiencies of the approaches described above. In one example, the issues described above may be addressed by a method for determining a rate of change of a FOS of an exhaust catalyst and a rate of change of an output voltage of an oxygen sensor arranged downstream of the catalyst. If a ratio of the rate of change of the FOS to the rate of change of the output voltage is positive, the method includes indicating an oxygen sensor fault and controlling engine operation independent of the oxygen sensor output voltage. Accordingly, a fault diagnosis of the HEGO sensor may be performed any time the engine is warmed up and operating in steady-state, in contrast to the intrusive approach described above in which the diagnosis can only be performed during certain engine operating conditions which do not occur frequently and often must be actively induced. Further, the methods and systems described in the present disclosure provide for an exceptionally robust diagnosis as they do not take into account the magnitudes of the parameters in question (e.g., the magnitude of the FOS and the magnitude of the HEGO sensor output voltage), but rather focus on the sign of the ratio of the rate of change of the FOS to the rate of change of the HEGO sensor output voltage. Thus, the technical effect of the methods and systems described herein is that a robust HEGO sensor fault diagnosis may be performed by monitoring the sign of the ratio of the rate of change of the FOS to the rate of change of the HEGO sensor output voltage.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.